1) Field of the Invention
In the various application cases contemplated, the track can have varying structures and be made of different materials, both metallic and non-metallic. In each case, the necessary pre-tensioning values can vary considerably. These depend on the weight and speed of the vehicle, its nature, the type of track installed, the environmental conditions in which the vehicle operates, on each occasion, and also other factors.
Analogously, the tension value limits allowable on the track also differ considerably and essentially depend on the maximum deflecting torque available, clearly linked to the weight of the vehicle, and also to its project dynamic capacities, such as the maximum values required relating to gradability, tensile stress or thrust allowance, in relation to the operating conditions and means of the vehicle.
The track-tightening device not only has the function of maintaining and regulating the tension established in the track while the machine is running. Crawlers are mainly destined to operate in so-called off-road environments, such as quarries, large civil work sites, landslide removals, open mines and so on. In this kind of use, it very frequently happens that a foreign body of any nature can enter the track unit. This is practically a fixed wheel base unit: as the track is an element with a fixed length allowance, i.e. not elastically extendable, the intrusion of a foreign body would require the lengthening of the track to also include the additional hindrance created. The intrusion thus suddenly induces a tension overload on the track and a parallel increase in stress on all the other mechanical units associated therewith, often exceeding the project limits, causing serious damage.
2) Description of Related Art
In practice, in order to overcome this problem, as the tracks are not elastically extendable, the track-tightening device also has the function of absorbing the tension due to the intrusion, by making it possible to shorten the wheel base i.e. by allowing an elastic-type back-pulling of the back-pull idler wheel. In this way a temporary excess is provided, to the necessary extent, of the length allowance of the track with respect to the temporarily reduced value of the crawler idler wheel base.
This not only allows the supplementary instantaneous hindrance of the foreign body inside the track unit to be tolerated and reduced but also limits the overtension induced.
In this temporary phase, by allowing an elastic reduction of the wheel base of the idler and crawler wheels which is sufficient to enable the track to be wound around the crawler wheel without any gripping on the part of its gear teeth, the transmission of the tensile stress from the crawler wheel to the track is eliminated, thus limiting the tension present to only that due to the thrust developed by the track-tightener in this phase.
This additional function attributed to the track-tightener means that it is also capable of having a temporary additional run swing for also tolerating and compensating the intrusion in the track unit of foreign bodies having significant and pre-determined dimensions.
In order to illustrate the technical problems faced and the advantages of the track-tightening device according to the present invention, FIG. 1 schematically indicates the overall undercarriage structure of a crawler, equipped for normal operating directions A and B.
The scheme of FIG. 1 illustrates the essential components of the “undercarriage” which consist of a jointed chain 1 which circulates between the crawler wheel 7 and the front wheel 3, with two substantially straight sections, a lower section defined by the lower wheels 2, on which the weight of the crawler is concentrated, and an upper section defined by the upper wheels 6, respectively.
In more frequent cases, the wheels 2 are individually fixed to the bogie; it often occurs however that the wheels 2 are assembled in pairs on rigid supporting elements, in turn fixed to the bogie. These elements can be of the equalizer type, capable of a limited angular rotation around a transversal axis to follow, with a clockwise and anticlockwise rotation, the unevenness of the ground, thus improving the adherence of the machine for a better contact between the track and the ground. These equalizers can also be equipped with a run end and elastic shock absorbers.
This form of embodiment, with rollers capable of oscillating, tends to cause a more complete faying surface of the track/ground contact point, on which the vehicle must move, with an improved traction/thrust capacity of the crawler. This type of arrangement of the supporting rollers 2 is normally defined as “floating bogie”.
A truck suspension assembly of this kind is known from U.S. Pat No. 3,826,325.
In U.S. Pat No. 3,826,325 the track frame of a vehicle is pivotally mounted to the vehicle main frame and has crank members pivotally mounted to the opposite ends of the track frame. Each crank member has mounted thereon an idler and a bogey to which is fixed a pair of track rollers. These idlers and track rollers are engaged by a continuous track chain, and the track frame is supported at the crank member-track frame pivot points by means of the idlers and track rollers. Stop members and resilient pad means are associated with the crank members, link members, and track frame for proper support of the track frame.
A series of soles or skids 4 is applied to the chain 1, which can be of various kinds and which must ensure both adherence to the ground to transmit the tensile strength and also the distribution and sustenance of the weight of the crawler on the ground. The shock absorber-tightening group 5 is connected to the axis of the front wheel 3, opposite the crawler wheel. This general scheme shows a traditional track-tightening device in which the idler wheel 3 is assembled on supports 8 which allow it to move in a longitudinal direction with respect to the undercarriage structure, the elastic shock absorber 9 is indicated as a propeller spring which operates under compression and a tightener as a grease cylinder 10. For the tightening and shock absorbing function, spring, oleodynamic, pneumatic devices have been proposed and also grease, floating piston devices integrated with each other by means of various structure either separate or integrated with each other each time, with solutions which satisfy all the individual performance requirements of the track-tightener, and also the necessity of limiting the hindrances of the overall system, facilitating assembly and maintenance.
In some cases, the scheme of FIG. 1 is modified according to the scheme of FIG. 1A, in which the back wheel 11 is a simple back-pull and the crawler wheel 7 is assembled in a backward but higher position, giving the perimeter of the chain 1 a triangular allowance.
In most crawlers, the structural supporting element of the weight of the machine to be distributed on the ground consists of an intermediate structure, generally called “bogie” or “undercarriage”, consisting of a kind of beam, present in a pair, at least one for each side of the vehicle, with a high rigidity and low deformability. In the most recent constructions, these structures are generally produced with boxes of a metallic carpentry, having a composite and/or lattice structure, on which the rolling elements necessary for moving and supporting the vehicle are assembled (in the scheme of FIG. 1 for example the supporting rollers 2 situated in the lower section of the chain 1), whereas the upper part of the “bogie” forms the rest base which supports all the remaining structure of the machine and its main elements, such as engines, drive, pumps and so forth.
If the crawler rests and runs on a substantially flat surface, there is no contribution nor requirement on the part of the idler wheel 3 for supporting the vehicle.
As a whole, the upper positioning of the idler wheel 3, with respect to the bearing line consisting of the tangent line connecting the supporting rollers 2 with the actual track itself (chain 1 and skids or soles 4) in the crawler, is defined so that the analogous contact point from the lower part of the idler wheel with the track is substantially higher than said tangent line. This arrangement is essential for the running regularity of the vehicle, allows a progressive grip with the ground on the part of the skid and also reduces the high resistance encountered by the track units when changing direction.
The situation on uneven land is, on the other hand, completely different: in the presence of a frontal obstacle or even only at the beginning of a steep slope it is the first part of the track carried by the front idler wheel 3 which supports the weight of the front part of the vehicle, whereas the supporting rollers 2 are suspended and cannot be used for supporting the vehicle. The element of the front idler wheel 3 has to carry part of the weight of the vehicle also in the presence of yielding or uneven ground.
Between the supporting rollers 2, normally all of the same size, and the idler wheel 3, there is generally a considerable difference in diameter and hindrance. This does not allow equalness in the minimum distance possible between the respective contact points of said elements with the chain 1 of the track: the elements with a smaller diameter can be distributed quite close to each other and create a series of contact points near to the track. On the contrary, the first supporting roller 2 of the line cannot be situated very close to the idler wheel 3, for the geometrical and mechanical reasons already explained. One of these is the diameter of the idler wheel itself 3, which is generally much greater than that of the supporting rollers 2 and which cannot be reduced to below a certain minimum value, in relation, in turn, to the pitch of the chain and maximum articulation angle possible between the rods or adjacent section of the chain itself.
The section of track between the axis of the idler wheel 3 and the axis of the first of the supporting rollers 2 is therefore the relatively longest part of track without contact points with the rolling elements. These parts are consequently more exposed to significant negative flexure deformation which considerably jeopardizes the integrity of the track system, also as a result of the deformations caused by the localized bumps and obstacles inevitably present in the ground, particularly in off-road use.
In this respect, the maximum allowable articulation in a negative sense on a track is normally equal to about ¾ of the corresponding value in a positive sense, i.e. of the articulation angle between two adjacent sections of the track when it is wound around the wheels 3 and 7. In general, a value of 8-10° is practically allowable as a maximum negative flexure angle, whereas the limit allowed as a positive flexure angle rises to 35-38°. In other words, the external periphery of the track should not have a significant concavity. In this case, exceeding the allowable negative flexure values causes mechanical interference phenomena among the adjacent elements forming the track (skids or soles 4) with the possibility of damage or breakage, considerable over-tensioning on the joints of the chain, and a slackening or loss of the skid fixing units, generally jeopardizing the integrity of the track. This situation is particularly serious in the case of chains of the lubricated type, which are of a high quality and costly but also extremely sensitive to ill-treatment.
Even if the external concavity of the track, or its negative flexure, remains within the allowable limits mentioned above, it is still a determining factor of the progressive internal wear of the track joints and reduction in its technical life. It is consequently essential that the negative flexure be limited and hindered as much as possible.
A reduction in the entity of this phenomenon has a considerable economical value, as a life prolonging factor of the track and consequently of the other parts forming the overall track unit and operating in contact therewith.
When determining the operating cost of crawlers, the amount relating to maintenance and depreciation of the tracks is generally one of the main items of its hourly cost, having the same importance as the other direct costs such as fuel consumption and labour cost. Prolonging the life of the track unit, with a consequent reduction in its cost incidence, is therefore determinant for competitiveness in the use of the vehicle. It should also be considered that the track unit must be completely replaced for reasons of wear after several thousands of operating hours, varying according to the type of machine.
Another function assigned to the track-tightening device is linked to the installation and subsequent tightening of the track on the machine. Considering the track is generally inextensible and must be fixed, to also resist transversal stress, to other elements of the track unit which prevent its uncoupling and side detachment when operating, its assembly without the track-tightening device would be practically impossible or in any case extremely difficult.
It should finally be taken into account that the regulation of the wheel base between the wheels is essential, during the life of the track, in order to “restore”, or recover, its extension with time, due to the progressive wear of its joints.
In operating practice, the track-tightener tends to compensate said extension by means of successive increases in the wheel base, until a total increase equal to ½ pitch of the track. At this point, one of the sections of the track is removed, thus bringing it to the original length allowance value, and parallelly the wheel base is brought back to its initial value.
In general practice, the various functions required of the track-tightener can be satisfactorily effected, as mentioned, by the combined action of an elastic-type element (for example a cylindrical propeller spring), with a device having a registerable run end function. It can consist for example in a cylinder/piston group containing grease under pressure, or with an oleodynamic drive. In simpler cases, it can be produced with a maneuvering screw device, with blockage by means of a nut and locknut; in more sophisticated applications, combined devices of the oleo-pneumatic type, have also been proposed, which are capable of effecting both shock absorbing and track-tightening functions.
Regardless of their constructive complexity, all these track-tightening systems require an instantaneous reduction of the elastic type in the wheel base between the wheels 3 and 7, when the tension value of the chain exceeds a set threshold value and in addition, and separately, the possibility of registering (for both increases and reductions) the wheel base between said wheels. The effect required is always a movement, depending on the forward or backward cases, of the back-pull wheel 3, which is typically an idler wheel.
From what is specified above, it is evident that with these movements of the back-pull wheel, the distance between the axis of the idler wheel 3 and the axis of the first supporting roller 2 of the bogie is also varied, which, according to the known art, causes considerable damage and drawbacks.